[Q] Converting an AT&T to T-Mobile. Help? topic

First off, thanks to everyone willing to read this and help me out. Here's the deal.

I got a cheap second-hand m8 locked on AT&T. So far I have sim-unlocked it, S-OFFed it, changed cid to 11111111(supercid), and added Philz recovery. I would like to find a firmware and rom for T-Mobile that has wifi-calling enabled, and flash it without bricking my phone. It still has the MID for AT&T/dev(0P6B120000). HBOOT version is 3.16.00000000.

Should I change the MID? What firmware can I flash? What roms are recommended? Do I need to update HBOOT?

I don't care about OTA's, once I am comfortable flashing roms on this thing, I'll do it that way.

I am coming from a Nexus 5 so I am familiar with the stupid easy methods for installing roms and radios and such and being impossible to brick. This phone seems to not be so easy, so I am being extra-careful with how I go about this, as it seems completely different to me. Also, some of the guides on here that I've read have had some conflicting and/or confusing information, not that I'm ungrateful, I just want to be absolutely sure before I trash my new phone.

Any help is appreciated. Thanks in advance. You guys are the greatest.